As the California Coastal Commission prepares to vote on the proposed convention center expansion Thursday, maps released by the Port of San Diego show that parts of the site could flood intermittently as sea level rises in the future.

Authorities with the port and commission said that while high tides and storms could spill seawater onto waterfront streets and walkways, they probably wouldn’t affect the building itself.

“The site level analysis anticipated minimal impact to the bayside roadway and promenade by year 2080 and no impacts to the expansion building by projected sea level rise,” port spokeswoman Tanya Castaneda said.

The analysis, prepared by Project Design Consultants and TerraCosta Consulting, examined how sea level rise expected as a result of climate change would combine with extreme high tide and wave action in the area planned for expansion.

Related

The port commissioned the study in the spring after Coastal Commission staff voiced concerns that issues of flooding and sea level rise weren’t fully explained in the project’s environmental studies.

The expansion proposal, in the works since 2008, would add 740,000 square feet to the 2.6-million-square-foot center. Proponents say it’s needed to retain big ticket events such as Comi-Con and to attract other large gatherings.

While the existing convention center is about 265 feet from the bay, expansion plans call for construction up to 70 feet from the waterfront. That proximity to the bay, along with projections for rising waters, increases the potential for flooding, planners said.

“Based on the location of the Convention Center Expansion, there are three flood hazard concerns that need to be considered – flooding from overtopping of the sea wall, flooding by backwater in the storm drain, and flooding by a tsunami,” a Coastal Commission staff report says. “The flooding conditions will all be worsened in the future with sea level rise.”

The analysis estimates that by the end of the building’s expected life span in 2080, sea level rise could add 2.53 feet to the mean water level in the bay. On top of that, an extreme high water event could add 5.32 feet of water, bringing levels up to 7.85 feet.

Those estimates are reasonable, said Scripps climate researcher Dan Cayan, whose work on sea level rise analysis has been used in regional projections, including a report on West Coast sea level rise by the National Research Council, one of the prestigious national academies.

“That’s certainly consistent with up to date thinking,” Cayan said of the analysis.

The existing sea wall ranges from 7.38 to 9.02 feet, the analysis stated. So at its lowest point, it could be breached by rising seawater.

Two- to three-foot waves from storms, tug boat traffic or distant tsunamis could bring water levels even higher, the report acknowledged. And storm drains could back up if water rose over their outfalls.

“The coincidence of all these factors happening at the optimum time to make really high water levels is kind of a rare event,” Cayan said. “Of course it’s a remote possibility, but it is a possibility.”

Even if that happened, it’s still not likely to flood the buildings, said Greg Shields, CEO of Project Design Consultants. Adding two to three feet of wave height would still be below the 12.3-foot elevation of the Convention Center floor, he said.

“Under the extreme high water, we wouldn’t anticipate any inundation” of structures, Shields said. “We would see portions of the promenade and portions of Convention Center Way inundated with some ponded water.”

Diana Lilly, a planner for the Coastal Commission, said the report satisfies the agency’s questions about effects of sea level rise on the proposed expansion.

“We looked at the analysis they did, and feel that they have adequately addressed it,” she said. “There probably will be some risk of flooding in the future, just as there is risk of flooding everywhere in the bay front, and this project won’t make it any worse.”

Nonetheless, she said, the larger question about waterfront development in an era of rising seas remains up for debate.

“Whether it’s ever a good idea to build something at that risk is a different question, and not one the Coastal Commission is addressing,” she said.